The trend in the food processing industry has been to replace metal cans with heat-sealed containers made from polymeric film for the packaging of certain food products stored under sterile conditions. Medical supplies are also often packaged and used in this manner. This packaging is known as a retort container. The containers may be flexible pouches or rigid trays.
Retort flexible pouches can be made of two sheets of flexible polymeric film heat-sealed on three sides before filling. Then after filling, and then vacuum drawing to remove oxygen from the inside of the pouch, the fourth side, the open-mouth end, is heat-sealed. Alternatively, the pouch can be made from a flexible polymeric tube of film where the bottom end is heat-sealed before filling and then after filling, the open-mouth end is heat-sealed. Also rigid coextruded trays or tubs can be made for use as retort containers.
The requirements of retort packaging include that the filled and sealed package be subjected to sterilizing conditions of high temperature and moisture, i.e. water or steam. Retorting can be from about 250.degree. F. (121.degree. C.) to 300.degree. F. (149.degree. C.) for about 10 minutes to 1 hour, and typical conditions are 275.degree. F. (135.degree. C.) for 0.5 hour, under water (or pressurized steam).
Polypropylene is known to be an excellent sheet material for retort pouches. It holds up well under the hot, moist retort conditions. However, since the pouch of food or medicaments after being retorted is typically stored on the shelf at ambient conditions, the retort pouch should prevent transmission of oxygen so that oxygen does not spoil the contents. Polypropylene has a high oxygen transmission rate. One answer has been to employ a laminate wherein a layer of the retort pouch is aluminum foil to act as a barrier to oxygen coming into the package. Aluminum foil has a zero oxygen transmission rate. A retort pouch having a layer of aluminum foil and a polymeric layer of polypropylene (herein abbreviated as PP) in a laminate is disclosed in U.S. Pat. No. 4,190,477 (1980), Ossian et al assignors to American Can. However, metal foil has three drawbacks. One is that it interferes with visual inspection of the enclosed product. The second is that metals cannot be used in a package for heating food in a microwave oven as the metal will spark in a microwave oven. The third is that metal foil laminates cannot be vacuum formed into rigid trays. An answer has been to make the retort pouch using a polymeric layer of a polymer that is a gas barrier polymer.
"Barrier" polymer refers to a property in some thermoplastic materials which indicates that the particular material has a very low permeability to gases, such as oxygen i.e. a low oxygen transmission rate. One barrier polymeric material is vinylidene chloride copolymer, designated as "PVDC". Vinylidene chloride copolymer is also commonly known as saran which has, in the United States, become generic and is not a registered trademark. Another known barrier polymeric material is acrylonitrile, herein abbreviated as AN. Another is hydrolyzed ethylene-vinyl acetate copolymer, which is also called saponified ethylene-vinyl acetate copolymer or ethylene-vinyl alcohol copolymer or hydrolyzed ethylene-vinyl acetate copolymer. This is designated by the abbreviations: "EVOH" or "HEVA". Sometimes it is referred to as "EVAL" which is a trademark, of Kuraray Co. Ltd. for EVOH.
A retortable package made entirely with polymeric materials and having a polymeric gas barrier layer of ethylene-vinyl alcohol copolymer is disclosed in U.S. Pat. No. 4,407,873 (1983), Christensen et al assignors to American Can. The film layers of the pouch are of the structure: (heat-sealing) linear low density polyethylene/blend of medium density polyethylene with linear low density polyethylene/anhydride modified medium density polyethylene/nylon/ethylene vinyl alcohol/nylon.
EVOH, however, is known to lose its oxygen barrier properties when subjected to moisture. The moist oxygen seeps into the package and spoils the contained food. In recent years, the requirements of the packaging industry have become increasingly demanding and for current commercial purposes, a permeability below 70 cc.mil thickness/m.sup.2.atmosphere.day at room temperature (which is equivalent to about 4.5 cc.mil thickness/100 in.sup.2.atmosphere.day at room temperature) is expected and a permeability below about 50 cc.mil thickness/m.sup.2.atm.day (about 3.2 cc mil thickness/100 in.sup.2.atm.day) is highly desirable. Even more preferably the permeability is below about 10 cc.mil thickness/m.sup.2.atm.day (about 0.64 cc.mil thickness/100 in.sup.2.atm.day). The test for oxygen transmission is conducted as per ASTM D3985.
Dow Chemical Corporation in its sales brochure entitled "Rigid Plastic Barrier Containers for Unrefrigerated Foods" describes the oxygen transmission rate (OTR) for some typical thermoplastic polymers as follows:
______________________________________ Oxygen Transmission Polymer cc.mil/100 in.sup.2.atm.day ______________________________________ PVDC 0.15 Nylon 66 2.0 Nylon 6 2.6 Polypropylene 150 EVOH 0.01 at 0% relative humidity EVOH 1.15 at 100% relative humidity ______________________________________
On page 14 of the brochure is a discussion of Dow's rigid tubs of coextruded sheet containing a saran layer for retort packaging. Dow's laboratory staff tested the oxygen permeability of several containers to illustrate the superior oxygen barrier properties under moisture of saran tubs as compared to EVOH tubs. The layers of the sheet were of the structures: PP/tie layer/EVOH/tie layer/PP and PP/tie layer/saran/tie layer/PP. (It is noted "tie layer" is another term for "adhesive layer".) The tub containers were filled with hot water, sealed, and retorted under water at 250.degree. F. (121.degree. C.) for 60 minutes at an air overpressure of 21 psig (2.5 kg/cm.sup.2). The retorted containers of both types were then emptied and tested for oxygen transmission. The oxygen transmission rate of the EVOH sheet was more than double that of the saran sheet. Furthermore, although the EVOH sheet dried out over time and its oxygen transmission rate decreased, it was still double that of the saran sheet. Clearly due to the retort moisture present, EVOH could not provide the low oxygen transmission rate that saran did.
Also of interest is U.S. Pat. No. 4,355,721 issued Oct. 26, 1982 to Knott et al, assignors to American Can, and U.S. Pat. No. 4,526,821 issued July 2, 1985 to McHenry et al, assignors to American Can. The former discloses flexible, multilayer polymeric retort containers having a core layer of EVOH and the latter discloses rigid, multilayer, polymeric retort containers having a core layer of EVOH. Both discuss the problems with the sensitivity, i.e. increase in O.sub.2 transmission rate, of EVOH when it is subjected to moisture.